Except that it isn't :)   Have you actually tried it?

Here's a simple example that mimics what's going on:

// foo.c
// represents the XS lib referencing a symbol (lgamma) in an external 
+library libm.so

#include <math.h>

double foo() {
    return lgamma(42.0);
}


// mytest.c
// this is just a wrapper which essentially does what perl/DynaLoader 
+is doing,
// i.e. loading the XS lib (Foo.so) dynamically at runtime

#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>

int main()
{
    double (*foo)(void);
    char *error;

    printf("dynamically loading 'Foo.so'...\n");
    
    void *so = dlopen("Foo.so", RTLD_LAZY);
    if (!so) {
        fprintf(stderr, "%s\n", dlerror());  exit(1);
    }
    dlerror();

    *(void **) (&foo) = dlsym(so, "foo");

    if ((error = dlerror()) != NULL)  {
        fprintf(stderr, "%s\n", error);  exit(1);
    }

    printf("%f\n", (*foo)());
    dlclose(so);
    exit(0);
}
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Run the following commands

gcc -c -fPIC foo.c                      # compile
gcc -shared -o Foo.so foo.o             # create/link shared library F
+oo.so

gcc -rdynamic -o mytest mytest.c -ldl   # create wrapper executable wh
+ich loads Foo.so
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Note that there are no errors while linking Foo.so, even though the command is missing the appropriate link instruction -lm referencing the math library!  As I pointed out, the shared library is created just fine.

Now, if you try to execute it you get the expected "undefined symbol" error at runtime:

$ LD_LIBRARY_PATH=. ./mytest
dynamically loading 'Foo.so'...
./mytest: symbol lookup error: ./Foo.so: undefined symbol: lgamma
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while if you link Foo.so with -lm (which corresponds to the step missing in the OP), everything works fine:

$ gcc -shared -o Foo.so foo.o  -lm
$ LD_LIBRARY_PATH=. ./mytest
dynamically loading 'Foo.so'...
114.034212
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